1. Field of the Invention
The invention relates to a filter apparatus for the uniform filtration of plastic melts, with a filter housing containing candle filters which each comprise a supporting body and a filter material and through which flows the plastic melt to be filtered.
2. Description of the Prior Art
A filter apparatus of this type is described, for example, in the periodical Kunststoff 70 [Plastics 70], year of publication 1980, No. II, pages 753 to 758, by F. Hensen and H. Simetzki. By means of filter apparatuses for plastic melts, impurities contained in the melt, such as dirt, gel particles, unmelted material and the like, are separated as efficiently as possible. Depending on the degree of purity required, mesh widths or pore widths of the filter material, which is screen netting, sintered material, nonwovens and the like, of 40 to 80 microns for polypropylene oriented films and 5 to 10 microns for polyterephthalate acid ester oriented films (polyester film or PETP film) are necessary for this purpose. These narrow mesh widths necessitate large filter surfaces to minimize the total pressure drop in the filter apparatus and the increase in the pressure drop in the course of time; on the other hand, a high total pressure drop and a sharp increase in the pressure drop in the course of time add to the costs of the filter process.
German Offenlegungsschrift No. 3,419,822 discloses a filter apparatus for screw-type extruders for the treatment of plastic melts having a filter housing with an inflow and an outflow bore and with an alternating slide displaceable in the latter bore transversely relative to the melt stream. At least one recess in the alternating slide receives a screening element. To ensure a uniform distribution of the melt stream in the filter apparatus and of the pressure forces exerted on the filter surface, the recess is annular, and there is a melt channel which communicates with the inflow bore and in which the cylindrical screening element surrounds the recess positively. A throughflow bore extending in the longitudinal axis of the alternating slide connects the inflow bore to the outflow bore in the filter housing. A displacement cone is installed in the throughflow bore, thus producing an annular collecting channel which widens towards the outflow bore in the direction of flow of the melt.
This known filter apparatus is a so-called filter pot, generally having a single filter surface of cylindrical shape which is arranged in the housing wall of the filter apparatus in the inflow and outflow bores. In the known filter apparatus, the aim is to achieve a uniform distribution of the melt stream in the filter and of the pressure forces exerted on the filter surface, and the complete elimination of transverse forces exerted on the guide surface of the alternating slide. The displacement cone in the melt channel is intended to reduce the flow resistance of the melt to a minimum in the alternating slide, while at the same time ensuring a crosssectional ratio favorable in flow terms.
From the point of view of the process used, it is desirable, when the filter surface is installed, to have a filter pot with as small a volume as possible and with no or only very small dead zones. In a filter pot, the filter surface generally forms the largest part of the pot circumference or housing wall, the external shape of the filter pot being similar to that of a cylindrical candle filter. If there are no or only very small dead zones in the filter pot, the dwelling-time distribution of the individual particles is short and the decomposition of the melt is minimal. The dwelling-time distribution describes what fraction of the mass particles have left the component after what time. The dwelling-time distribution of the melt particles in a pipe through which the flow passes can be calculated by integrating the speed of the individual particles over the cross section of the pipe. As is known, the speed near the pipe wall is virtually zero and increases to the maximum value towards the center of the pipe. The narrowness of the dwelling-time distribution refers to the fact that over the full pipe cross section the speed differences of the particles should be as small as possible, that is to say, the dwelling times of the melt particles should have as little differences as possible over the pipe cross section, and this can only be achieved if the speed distribution of the particles is flattened out to a great extent, this only occurring when the speed maximum levels off sharply in the center of the pipe.
This means, in other words, that the rise of the time curve representing the fraction of particles which have left the component per unit of time is very steep.
A preliminary condition for ensuring that the dead times are as little as possible is that the melt must flow through the complete filter volume. This is easiest to achieve if, within the filter pot, the speed vector changes only very slightly or not at all in terms of both amount and direction. A constructive design of the filter apparatus, in which, for example, the melt flow is deflected 90.degree. to 180.degree., should be avoided if possible.
According to the present state of the art described, for example, in the literature references "Filtrieren von Kunststoffschmelzen" ["Filtration of plastic melts" ], VDI Verlag GmbH, 1981, Kunststoff 70 [Plastics 70], year of publication 1980, No. II, pages 753 to 758, and Plastverarbeiter [Plastics Worker], Volume 33 1982, No. 12, pages 1447 to 1454, compact filter apparatuses meeting the above-mentioned requirements can best be provided by arranging filter inserts of candle or discus shape concentrically in filter housings. To obtain a compact design in a filter apparatus with candle filters, the candle filters are arranged with their center points generally on concentric circles, starting from the center of the filter housing where a central candle filter is located. The flow through the filter apparatus takes place without any substantial deflection of the melt stream which, for example, flows in from above and flows out of the filter housing again at the bottom. In this known filter apparatus, the dwelling-time distribution is not sufficiently narrow, and the length of the filter service life still leaves much to be desired.